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For Earth observation satellites such as SPOT, it is important to measure the modulation transfer function (MTF) in order to quantify the spatial resolution of the imaging system. This measurement is useful to decide to focus the telescope or to make a deconvolution filter whose purpose is to enhance image contrast by ground processing. The paper presents an MTF measurement method used during the in-orbit checkout of SPOT3 and the corresponding results. This method relies on a spotlight pointed to the satellite. The image of the spotlight nearly corresponds to the point spread function (PSF) of the system, but this PSF is undersampled because the sampling rate is limited by the pixel spacing. The samples are too sparse to describe the PSF completely and to obtain the MTF directly by computing the PSF Fourier Transform. Indeed, aliasing falsifies the computations. At this step, unless several spotlights are used, the use of an MTF model is mandatory: the model, including a parametric defocus, enables computation of the PSF at a high sampling rate and then the problem is to match the measured samples to the samples derived from the model. The MTF of the system corresponds to the MTF of the model which leads to the best matching. The paper reviews the context of work, the measurement method and, for SPOT3, the MTF model used, the experimentation and the results obtained.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>